1. Field of the Invention
The present invention relates to a semiconductor storage device and, more particularly, to a semiconductor storage device provided with a non-volatile memory having a floating gate.
2. Description of the Related Art
In recent years, demand for a non-volatile memory in which stored data is held even after the power is turned off is increasing. Especially, demand on an EEPROM (Electrically Erasable Read-Only Memory) is increasing. A cell structure of a conventional EEPROM will be described with reference to FIGS. 7(a) to 7(c) and FIG. 8. The EEPROM comprises a source 12, a drain 11, a floating gate 3 for accumulating charges, a gate insulating film (tunnel oxide film) 5 formed between the floating gate and a well, an ONO film (insulating film) 4 which is a stacked layer of an oxide film--a nitride film--an oxide film formed between a control gate and the floating gate, and a side wall 13 covering the side faces of the floating gate and the control gate. As shown in a cross section of FIG. 8 taken along the line A-A', the side faces of the floating gate 3 and the control gate 5 are covered with a thermal oxide film 13-1 having a thickness of 100 angstroms or larger, the thermal oxide film 13-1 is covered with a hot thermal oxide (HTO) film 13-2, and the HTO film 13-2 is covered with a CVD oxide film 13-3 deposited by chemical vapor deposition (CVD). Since electrons in the floating gate 3 are leaked and the data holding characteristics deteriorate when only the HTO film 13-2 and the CVD oxide film 13-3 are used, the thermal oxide film is formed during heat treatment to improve the film quality of the HTO film 13-2 and to improve the data holding characteristics. The thermal oxide film is formed by diffusing oxygen into the floating and control gates via the HTO film 13-2.
The problems of the conventional EEPROM are as follows. A gate insulating film 2 under the floating gate 3 is also subjected to oxidization similarly due to the heat of thermal oxidation used to form the side wall 13. As a result, what is called a gate bird's beak such that a tunnel film as the gate oxide film 2 near the edge of the floating gate 3 becomes thick occurs. A Fowler-Nordhein current which becomes larger as the drain impurity concentration becomes higher and whose current density becomes higher as the tunnel film becomes thinner is not passed near the edge of the floating gate where the drain impurity concentration is high and the tunnel film is thin. The Fowler-Nordhein tunneling phenomenon (FN tunneling phenomenon) does not therefore contribute to writing. Consequently, a first problem such that the speed of electrons moving from the floating gate to the drain decreases occurs. A second problem is as follows. The FN tunneling current is not passed around the edge of the floating gate 3, the electrons are moved only from a part of the tunnel film, and the amount of electrons passing the tunnel film per unit area increases, so that tolerance to the number of rewriting times deteriorates. As a third problem, since the tunnel film near the edge at which the FN phenomenon that the electrons can be moved with a small amount of the writing current occurs becomes thicker, the interband tunneling current becomes dominant and the writing efficiency (FN current/current consumed at the occasion of writing) deteriorates.